Respiratory screening and diagnosis is an important part of medicine and health. Incidences of asthma, chronic obstructive pulmonary disease, and emphysema have grown to epidemic proportions in the last few years. Hospital ventilators are being used more and more for life support for the very young, very old, and all those in between. Traditional pulmonary function tests are not always adequate to provide the respiratory resistance information required by health care professionals. The Airflow Perturbation Device (APD) has been under development for many years, and has undergone significant improvement. The APD is relatively simple in concept: it uses a rotating wheel in the flow path to change flow and mouth pressure by small amounts. From these changes, resistance can be calculated. The APD is simple in operation: it requires only normal breathing for a short time, and can be used by the very young as well as adults. It can show resistance changes as they occur. It can be modified to measure resistance of ventilated patients and can be made as small and portable as a camera. The APD in present form has been used on nearly 1000 people. Normal values of average respiratory resistance have been determined. Resistances in the inhalation and exhalation directions have also been obtained. The APD has been used to find that adhesive nasal strips can reduce nasal resistance by about 10%, and it has been used to demonstrate the transient reduction of resistance following exercise. It should also be able to help titrate bronchodilator doses in hospital patients. In order to bring the APD into commercial production, some further refinements are necessary. After that, clinical data must be obtained, and regulatory approval must be sought. The market for the APD is huge, including all hospitals, clinics, physician's offices, ambulances, and home care facilities in the world. It can also be used in veterinary settings. In Phase I work we plan to further develop the system to be battery-operated, low weight, portable, and less expensive. It will be clinically tested on at least 200 individuals.